In appearance, runners look like driblets of lava
running down the wall of the cave. They are actually a form of tubular
lava stalactite formed along a surface instead of free-hanging. They
may also form along the edge of a stalactite. Like tubular lava stalactites,
they are formed of cooling segregated lava that is extruded from semi-crystalline
wall matrices, propelled by boiling and escaping gases.